Balkan Peninsula, peninsula in southeastern Europe, bounded on the east by the Black and Aegean seas, on the south by the Mediterranean Sea, and on the west by the Adriatic and Ionian seas. The Balkan Peninsula generally encompasses the countries that are commonly known as the Balkan states: Croatia, Bosnia and Herzegovina, the Former Yugoslav Republic of Macedonia, Serbia, Montenegro, Albania, Greece, and Bulgaria. In addition, the southeastern part of the peninsula includes Eastern Thrace, the European part of Turkey. Geographically, the northern boundary of the peninsula can be defined by the Sava River; the lower Danube River from the point, at Belgrade in Serbia, where the Sava joins it; and a line drawn arbitrarily from the upper Sava to the Adriatic Sea near Rijeka, Croatia. This northern boundary encompasses a region that (together with Romania and excluding Montenegro, Dalmatia, and the Ionian Islands) constituted most of the European territory of the Ottoman Empire from the late 15th to the 19th century.Most of the Balkan Peninsula is mountainous, with streams flowing in every direction. The drainage area of the Danube is the most important hydrographic feature. From the southern slopes of the Rhodope Mountains, the Vardar (Axi?s) River flows into the Aegean Sea; and, at the most westerly point of the Balkan Mountains, the Morava flows into the Danube. The Balkan Mountains form the largest continuous range; other mountainous sections are the so-called Dinaric Alps along the Adriatic coast, the Rhodope chain between the Former Yugoslav Republic of Macedonia and the Maritsa Valley, the Pindus range in northwestern Greece, and isolated summits of historical importance, including Mounts Olympus, Pelion, and ?ssa in Greece. Lake Scutari, on the Montenegro-Albania border, and Lake Ohrid, on the border between Albania and the Former Yugoslav Republic of Macedonia, are the only lakes of importance on the peninsula. The southern part of the peninsula, which forms the mainland of Greece, has a mild Mediterranean climate, but the rest of the region is subject to the severe winters and hot summers of south-central Europe. The northwestern portion has few lowlands and is characterized by unbroken, jagged hills; the southern portion has much more level terrain.

The first peoples to migrate into the Balkans arrived from all directions: central Europe, southern Europe, Asia, and Asia Minor. During the rule of the Roman and Eastern Roman (Byzantine) empires (1st century until the late Middle Ages), the region was periodically invaded by various ethnic groups. The Slavs appeared around the 3rd century and migrated to the peninsula in large numbers in the 6th century, and Bulgar tribes appeared in the 7th century; eventually the Bulgars were assimilated by the Slavs. The Slavic and various other ethnic settlers evolved in comparative isolation because of the natural barriers to communication, each group developing its own culture, customs, and religion.

Balkan history is characterized by military and political strife. Because the peninsula is politically and economically important as part of the land bridge between Europe and Asia and the overland route from the Mediterranean to the Black Sea, it was subjugated for centuries by a series of conquerors. Conflict between Balkan peoples and nations has been common, and the Balkans have played a key role in European power struggles. The Balkans once were Roman provinces and the peninsula remained part of the Byzantine Empire until the late Middle Ages, when the Ottoman Turks invaded and gradually took control of almost the entire peninsula.